[gdb/testsuite] Fix xfail handling in gdb.threads/gcore-thread.exp

When running test-case gdb.threads/gcore-thread.exp on openSUSE Tumbleweed,
I run into these XFAILs:
...
XFAIL: gdb.threads/gcore-thread.exp: clear __stack_user.next
XFAIL: gdb.threads/gcore-thread.exp: clear stack_used.next
...

Apart from the xfail, the test-case also sets core0file to "":
...
        -re "No symbol \"${symbol}\" in current context\\.\r\n$gdb_prompt $" {
            xfail $test
            # Do not do the verification.
            set core0file ""
        }
...

After which we run into this FAIL, because gdb_core_cmd fails to load a
core file called "":
...
(gdb) core ^M
No core file now.^M
(gdb) FAIL: gdb.threads/gcore-thread.exp: core0file: \
  re-load generated corefile
...

Fix this FAIL by skipping gdb_core_cmd if the core file is "".

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-04-06  Tom de Vries  <tdevries@suse.de>

	PR testsuite/27691
	* gdb.threads/gcore-thread.exp: Don't call gdb_core_cmd with core
	file "".

1 files changed, 16 insertions, 4 deletions

diff --git a/gdb/testsuite/gdb.threads/gcore-thread.exp b/gdb/testsuite/gdb.threads/gcore-thread.exp
index 2a6ab17..35c75f7 100644
--- a/gdb/testsuite/gdb.threads/gcore-thread.exp
+++ b/gdb/testsuite/gdb.threads/gcore-thread.exp
@@ -114,11 +114,13 @@ if {"$core0file" != ""} {
 # Now restart gdb and load the corefile.
 clean_restart ${testfile}
 
-foreach name { corefile core0file } { with_test_prefix $name {
-    set core_loaded [gdb_core_cmd [subst $$name] "re-load generated corefile"]
+proc load_core { filename } {
+    global horiz nl
+
+    set core_loaded [gdb_core_cmd $filename "re-load generated corefile"]
     if { $core_loaded == -1 } {
 	# No use proceeding from here.
-	continue
+	return
     }
 
     # FIXME: now what can we test about the thread state?
@@ -139,4 +141,14 @@ foreach name { corefile core0file } { with_test_prefix $name {
 
     gdb_test "info threads" "\\* ${horiz} thread2 .*${nl}" \
 	    "thread2 is current thread in corefile"
-}}
+}
+
+foreach name { corefile core0file } {
+    set filename [subst $$name]
+    if { $filename == "" } {
+	continue
+    }
+    with_test_prefix $name {
+	load_core $filename
+    }
+}